Combustion gases from incinerators, power plants, and coal-fired furnaces typically contain oxides of sulfur (SOx), oxides of nitrogen (NOx), and volatile heavy metals such as mercury. Typical mercury concentrations in coal are 0.05 to 0.25 mg/Kg. On combustion, the mercury is volatilized and carried in the combustion exhaust gases.
Mercury poses a serious problem for human beings and the environment. MSNBC.com recently reported that the Southeast United States alone could save up to $2 billion a year by reducing mercury pollution. Coal-burning electric power plants are the single biggest source of mercury emissions, accounting for 40 percent of the total. Coal-fired burners account for another 10 percent.
Prior efforts to control SOx, NOx, and mercury emissions have included the use of dispersions of calcium carbonate and active carbon as an injected spray administered to exhaust gases, and/or the passing of exhaust gases through a scrubbing tower to neutralize the SOx and NOx emissions and sorb the volatile mercury. Following the scrubbing procedure, solids carried in the exhaust can then be recovered by electrostatic precipitators prior to discharge of the gases to the atmosphere. A combination of scrubbing and electrostatic precipitators used to condense gas-bound dusts can typically remove 50-85% of the gas-borne mercury.
Other efforts to control mercury emissions have employed alkali metal sulfides, e.g. sodium polysulfide solution, and in particular sodium tetrasulfide solution. See, e.g., U.S. Pat. No. 6,214,304, and Babcock Power Environmental Inc. Technical Publication, “Multi-Pollutant Emissions Control & Strategies, Coal-Fired Power Plant Mercury Control by Injecting Sodium Tetrasulfide”; Licata A, Beittel R, Ake T, ICAC Forum, Nashville, Tenn. Oct. 14-15 2003. Mercury is converted to mercury sulfide, and the mercury sulfide is precipitated out via a dust separator. The process requires the alkali metal sulfide solution to be introduced into the flue gas accompanied by the simultaneous addition of heat. The process can be used in conjunction with the addition of oxidizing agents, e.g. chlorine-containing compounds, to the burning coal or coke, such that elemental mercury is converted into its oxidized form, enabling the reagent to react with it more readily. This process is disadvantageous in that it is temperature-critical and the use of oxidizing agents in coal feedstocks may increase corrosion within the system. Additionally, the process residues containing precipitated mercury sulfide, in its black or beta-crystalline form, can be readily oxidized, as mercury sulfide is not stabilized to pH- or redox-induced reactions, thereby rendering the mercury sulfide precipitate vulnerable to re-oxidation and resolubilization in water, with the potential for increased bioavailability.